To Don regarding design for a 46 button Hayden Duet.
The 46 button Wheatstone Hayden Duet is the standard 6.25". It of course uses traditional concertina reeds set radially, with 21 buttons on the left and 25 on the right.
The Crabb 45 button Salvation Army Crane duet is only 6" across. The traditional concertina reeds are set up and down as is common on Crabb concertinas. Left hand side - 20 buttons, 8 up, 8 down, and 4 in the middle; but there is enough space to put an extra pair of reeds on the right hand side of the left hand reedpan. Right hand side - 25 buttons, 9 up, 9 down, 5 in the middle, and 2 sideways (one at each end of the reedpan). I tried one of these which had been very much used and did not notice any difference in the timbre of the edge and centre pan notes.
I have had concertinas from new which have some reeds on the centre of the reed pan. The difference in timbre was quite noticeable when brand new, but this has settled down to be unnoticeable after a good time of playing. These concertinas all had the centre reeds properly routed into the centre of the reed pan, not screwed in.
With rectangular accordion reedplates all screwed onto the equivalent of a reedpan, I cannot see why there should be any difference in timbre at all.
Concertina Makers do not usually publish concertina designs. It takes a lot of time to design a new type of concertina, and the makers consider that the design is their copyright.
Now let us consider how many buttons we might be able to get into a small size concertina.
Accordion reedplates are a pretty standard 16mm across and lengths according to the pitch - see Harmonikas CZ charts. Some years ago I designed one using the well established spacing for a 6.25" concertina. 8 X 16mm (128mm): plus 2mm between the plates (to allow them to be waxed in), and 2mm at each side (18mm): plus two 6mm bellows frame sides (12mm); gives 158mm: which is 6.25" as near as makes no difference. I designed one such Hayden Duet but was only able to get 23 reedplates in the right hand side, but the loss of the highest c#"' & d"' was I felt no great loss.
However if you went wall to wall with screwed on reedplates, it might be possible to get 9 reedplates across on a 6.25" concertina. With the slightly larger 6 5/8" size bellows, 9 reedplates across should be no problem; and that should easily take all 46 buttons of the common small Hayden Duet
Well best of luck with your venture, I eagerly await to see the result.